Variable speed fan motor control for forced air heating/cooling system

ABSTRACT

A fan motor speed control system for controlling the fan motor speed of an air conditioning system includes a power output circuit including a power triac which is turned on and off by an opto-isolator connected to a pulse generator circuit for varying an AC voltage waveform imposed on the fan motor. The pulse generator circuit is connected to heating and cooling ramp circuits and a minimum speed circuit to provide a variable voltage signal imposed on the pulse generator circuit corresponding to the temperature difference sensed by a return air sensor and a heating or cooling sensor or by separate heating and cooling sensors disposed adjacent respective heating and cooling heat exchangers of the air conditioning system. An adjustable minimum speed circuit and a cutoff circuit are provided to control motor minimum speed or motor shutoff when a predetermined minimum speed is reached to prevent motor bearing failure or overheating. Sensor protection circuits in the control system operate to drive the motor to full speed if any of the temperature sensors experience an open or short circuit condition. The control system circuit maximizes air conditioning system efficiency by capturing additional heating or cooling effect, reduces noise associated with motor startup and shutdown, and reduces rapid change in the sensed temperature in the air conditioned space during motor startup and shutdown.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.09/629,328, filed Aug. 1, 2000, now U.S. Pat. No. 6,695,046, issued Feb.24, 2004, which is a continuation-in-part of U.S. patent applicationSer. No. 09/570,880, filed May 15, 2000, now U.S. Pat. No. 6,684,944,issued Feb. 3, 2004 which is a continuation of U.S. patent applicationSer. No. 08/801,560, filed Feb. 18, 1997, now U.S. Pat. No. 6,070,660,issued on Jun. 6, 2000.

FIELD OF THE INVENTION

The present invention pertains to a control system for continuouslyvarying the speed of a fan drive motor for a forced air indoor spaceheating/cooling system during startup and after shutdown of aheating/cooling cycle.

BACKGROUND OF THE INVENTION

Conventional controls for forced air heating and cooling systems oftenprovide for delayed startup of the fan drive motor at a single operatingspeed and delayed shutdown of the drive motor from a single operatingspeed after shutdown of the heat exchangers of the heating/coolingsystem. Conventional controls are designed to minimize unpleasant coldor hot drafts of air and to capture residual heat/cooling effect.However, changing motor speed abruptly from a deenergized or shutoffstate to full speed usually generates unpleasant noise, does notpreclude stratification of air in the system ductwork or in the spacebeing heated or cooled, nor does such operation maximize the capture ofresidual heat/cooling effect of the system heat exchange equipment.

Control systems have been developed for forced air heating/coolingsystems wherein the indoor space air circulating fan drive motor isdriven at reduced speed for a period of time during startup and at areduced speed for a period of time during the run-on or shutdown phaseof the heating/cooling system operating cycle. Again, this type ofcontrol system does not minimize the stratification of warm or cold airin the ductwork or the space being heated or cooled nor does such asystem maximize the capture of residual heating/cooling effect.

Prior patent applications Ser. Nos. 09/570,880 and 08/801,560 (now U.S.Pat. No. 6,070,660) assigned to the assignee of the present inventionand referenced hereinabove are directed to an improved fan or blowerdrive motor control system and method for forced air heating/coolingsystems wherein the fan drive motor speed is continuously varied duringa starting phase and a shutdown phase of operation of theheating/cooling system. In one embodiment of the control systemdisclosed in the aforementioned patent application and patent, thesystem senses temperature in the airflow circuit of the heating/coolingsystem and prevents premature or unwanted operation of the fan drivemotor. The present invention is directed to improvements in controlsystems of that general type. The subject matter of U.S. Pat. No.6,070,660 issued Jun. 6, 2000 to Howard P. Byrnes, et al. isincorporated herein by reference, in its entirety.

SUMMARY OF THE INVENTION

The present invention provides an improved fan or blower drive motorcontrol system for a forced air heating/cooling system wherein a controlcircuit is provided which substantially continuously varies the speed ofthe fan drive motor during a starting phase and a shutdown phase ofoperation. The control system may be easily adapted to conventionalheating/cooling system controls to vary the forced air fan or blowerdrive motor speed in response to temperatures sensed in theheating/cooling system airflow circuit. The control system isparticularly adapted for but not limited to use with permanent splitcapacitor or shaded pole blower or fan drive motors.

The control circuit includes an onboard power supply, an ac voltage wavecrossover detector circuit and a control circuit for firing a triac tocontrol the drive motor speed. The control system also includes aminimum speed detector circuit and a circuit which provides forcontinued operation of the fan drive motor at the minimum speed, ifdesired, or motor shutoff after reaching the minimum speed.

The control system of the present invention includes one embodimentwhich comprises a temperature sensor disposed in an airflow ductwork onthe so-called return air side of the heating and/or cooling equipmentand a temperature sensor on the downstream or so-called supply air sideof the heating and/or cooling equipment.

In another embodiment, three sensors are disposed in the ductworkincluding the return air sensor which is disposed upstream with regardto the direction of airflow from an air heater heat exchanger, a heatsensing sensor which is disposed downstream of the air heater heatexchanger and a third sensor which is disposed downstream of an aircooling heat exchanger, such as an evaporator coil, for example. In thisway a more versatile control system is provided and more accuratesensing of temperature is obtained, depending on the operating conditionof the system, heating versus cooling.

The control systems of the present invention advantageously reduceenergy consumption of conventional forced air heating and coolingsystems, improve recovery of residual heating/cooling effect inconventional forced air heating/cooling systems, minimize stratificationof air in the airflow circuit and the space being heated or cooled andreduce cold or hot air drafts during operation of the heating/coolingsystem. Moreover, by substantially continuously varying the fan orblower drive motor speed during startup and shutdown, noise associatedwith fan or blower operation is reduced and the circulation of air at atemperature other than normally sensed or preferred by occupants of anindoor space being heated or cooled is also reduced.

Those skilled in the art will further appreciate the important featuresand advantages of the invention, together with other superior aspectsthereof upon reading the detailed description which follows inconjunction with the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of air temperature versus flow and motor speedindicating the change in airflow with increasing temperature sensed inthe airflow circuit as well as decreasing flow with decreasingtemperature in the airflow circuit in accordance with the control systemof the present invention;

FIG. 2 is a schematic diagram of one preferred embodiment of a controlsystem in accordance with the invention; and

FIG. 3 is a schematic diagram of another preferred embodiment of theinvention and comprises FIGS. 3A, 3B and 3C, which may be viewed whenarranged in accordance with the map diagram of FIG. 3; and

FIG. 4 is a somewhat schematic illustration of an air conditioningsystem showing one preferred arrangement of the locations of the sensorsfor the control system of FIG. 3.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the description which follows like elements are marked throughout thespecification and drawing with the same reference numerals,respectively. Conventional elements are shown in somewhat generalized orschematic form in the interest of clarity and conciseness.

Referring to FIG. 1, the diagram illustrates a preferred change in motorspeed and airflow rate through a conventional forced air heating/coolingsystem when the system thermostat senses the need for heating, forexample, at a temperature setpoint of 78° F. in the space being heated.When the temperature sensed by the conventional system temperaturesensor or thermostat drops below the setpoint of 78° F., for example,the furnace or heater turns on and the control system of the inventionenergizes the blower or fan drive motor at a minimum speed. When the airtemperature sensed in the system ductwork increases, primarily at alocation just downstream of the heater heat exchanger, as compared withthe temperature in the return air duct upstream of the heater heatexchanger, the motor speed is increased.

Motor speed is proportional to airflow increase, and generally followscurve 10 in FIG. 1 until the temperature sensed by the sensor which isdownstream of the heater heat exchanger reaches a setpoint of 110° F. Atthis time the blower motor continues to operate at full speed until thethermostat in the space being heated indicates that the demand forheating has been satisfied and the heater or “burner” is turned off sothat the heater heat exchanger begins to cool. Accordingly, as thetemperature sensed by the sensor which is disposed downstream in thedirection of flow of air through the system decreases in relation to thereturn air temperature sensed at a point upstream of the heat exchanger,the control system of the invention varies the fan speed by continuouslydecreasing the fan drive motor speed. Airflow provided by the motordriven fan decreases along curve 12 in FIG. 1 until a minimum speed ofthe motor is reached which may result in continuous operation at theminimum speed or, at a slightly lower temperature, motor shutoff occurs.

Accordingly, motor operation and the airflow characteristic, as afunction of the sensed, temperature, provides for delivery of residualheat from the heater heat exchanger to the space being heated withincreased efficiency, airflow increases and decreases gradually on startand stop of the heater or burner for quiet operation of the system andstratified air layers at various temperatures are substantiallyeliminated in the heating/cooling system ductwork and in the space beingheated or cooled. More efficient operation of the heating/cooling systemis obtained and a greater comfort level is provided for personsoccupying the space controlled by a system in accordance with theinvention.

Referring now to FIG. 2, there is illustrated a schematic diagram of onepreferred embodiment of a temperature sensing, variable speed fan orblower motor control system in accordance with the invention andgenerally designated by the numeral 20. The control system 20 isoperable to sense the temperature in a ductwork of a conventional forcedair heating and cooling system, a section of which ductwork isillustrated in FIG. 2 and generally designated by the numeral 22.Ductwork 22 includes a return air duct part 24 whereby airflow from aspace being heated or cooled is being returned for heating by a heaterheat exchanger 26 or cooled by a cooling heat exchanger 28. Accordingly,a return air temperature sensor R11 is disposed in the ductwork 22upstream of the heat exchanger for the heater 26 and a so-called supplyair sensor R9 is disposed in ductwork 22 downstream, with respect to thedirection of airflow, of the air cooling heat exchanger 28 wherebysupply air treated by the heating/cooling system is then returned to thecontrol space via a supply air duct 30. In fact, the ductwork 22 maycomprise a conventional forced air furnace/air conditioning systemwherein the heat exchanger 26 includes a gas fired burner or electricalresistance heater, not shown, and the heat exchanger 28 is an evaporatorcoil of a conventional vapor compression refrigeration circuit, notshown. The illustration of FIG. 2 with respect to the heating/coolingsystem is exemplary.

Referring further to FIG. 2, a HEAT/COOL SELECT circuit is indicatedwhereby, for example, when a heater associated with heat exchanger 26 isenergized, such as by opening a gas burner valve, for example, 24 voltAC electrical power is applied across terminals P5 and P6.Alternatively, when an air cooling system is operable, such as a vaporcompression refrigeration system, and the compressor thereof isenergized, 24 volt AC power is applied across terminals P4 and P6. Powerfor the control system 20 is supplied by a 120 volt AC source atterminal P2 and a neutral conductor P2′. Alternatively, 24 volt AC powermay be applied at terminals P7 and P2′. A fan or blower drive motor 32may be connected at terminals P2′ and P2″ as indicated in FIG. 2. Themotor 32 may be of a type described in U.S. Pat. No. 6,070,660 which isincorporated herein by reference. The control system 20 is preferablyconnected to the motor medium speed winding as in the system of the '660patent.

As further shown in FIG. 2, a 12 volt DC power supply circuit is made upof capacitors C2 and C10, resistor R15, a diode D2 and a Zener diode D5.A four diode bridge BR1 takes either the 24 volt AC signal from a stepdown transformer, not shown, or the 120 volt AC source at terminals P2and P2′. A RESET circuit comprising resistors R21, R24, R25, R26, R27,R44, diodes D4 and D6, capacitor C4 and amplifier U2:A is operable toreceive full wave voltage from the diode bridge BR1 through resistorsR25, R26, diode D6 and amplifier U2:A to capacitor C6 for the purpose ofdischarging capacitor C6 every half cycle. Thus an output pulse alwaysstarts at the proper moment on each half cycle. If 24 volt AC power isinput to the power supply and RESET circuits, jumper JP2 is open and isshorted if there is no 24 volt AC supply.

Sensors R9 and R11 are preferably thermistors which are substantiallysimilar and interposed in a HEAT/COOL RAMP GEN circuit to generatesignals as the temperature differences change between each sensorlocation. If both sensors are at the same temperature the output of thesensors will be one-half of the 12 volt DC supply voltage. If thedownstream or so called supply air sensor R9 senses a temperaturegreater than the return air sensor R11, the output voltage at conductor34 increases. If the temperature sensed by sensor R9 is less than thatsensed by sensor R11, voltage at conductor 34 will decrease. The outputsignal from the sensors R9 and R11 is input to the ramp circuitsindicated in FIG. 2 as the COOL RAMP and the HEAT RAMP. If the outputsignal voltage is increasing the HEAT RAMP circuit is activated whichcomprises resistors R10, R12, R13, R14, R16, R17, R18 and R20,capacitors C8 and C13, diode D3, buffer amplifier U1:C and amplifierU1:A arranged in circuit as shown in FIG. 2. The output signal of theHEAT RAMP circuit is imposed on conductor 36.

The COOL RAMP circuit is also connected to conductor 34 to receive theresultant output signal from sensors R9 and R11 and if the signalmagnitude is decreasing, a voltage output at conductor 36, 37 isincreasing. The ramp output voltage generated by the COOL RAMP circuitis provided by circuit components including resistors R1, R2, R3, R4,R5, R6, R7, R8, capacitor C14, diode D1 and amplifiers U1:B and U1:D.Capacitors C13 and C14 slow the change in the output signal of amplifierU1:B or U1:C which will minimize the chance of lockup of motor 32.Capacitors C13 and C14 also minimize unwanted electrical noise fromentering the ramp circuits previously described.

The control system 20 further includes a pulse generator or PULSE GENcircuit including resistors R35, R36, R37, capacitors C5, C6 and C9,opto-isolator U3 and diode D10. Ramp output voltage is input throughresistors R19 and R28 to the PULSE GEN circuit and operational amplifierU2:B which has a reference voltage set at its negative input. When theramp voltage exceeds this reference voltage, the output of amplifierU2:B goes “high”. Capacitor C6 connects to the ramp voltage signal onconductor 36 also. Accordingly, a sawtooth waveform is input at thepositive (+) terminal of amplifier U2:B. Therefore, the output of thePULSE GEN circuit is a square wave whose width varies as the rampvoltage signal varies. Since the RESET circuit discharges capacitor C6every half cycle, the output pulse of the PULSE GEN circuit alwaysstarts at the correct time on each half cycle.

A POWER OUTPUT circuit is shown in FIG. 2 comprising resistors R30, R32,power triac Q1 and capacitor C7. A square wave output signal from thePULSE GEN circuit is imposed on capacitor C5 which causes a voltagepulse to turn on the input diode of opto-isolator U3 and when the diodein opto-isolator U3 conducts its output triac turns “on”. This actioncauses current to flow into the gate of the power triac Q1 which isconnected to motor 32. When current flows through the power triac Q1,motor 32 is energized to rotate to drive fan or blower 33 which isoperably associated with the ductwork 22. A snubber comprising resistorR32 and capacitor C7 are connected to power triac Q1 to protect triac Q1from unexpected line voltage surges.

Referring still further to FIG. 2, a CUTOFF circuit includes resistorsR33, R34, R38, R39 and R40, diode D8 and amplifier U2:C. Amplifier U2:Cis operable to receive a variable voltage signal at its negativeterminal via conductor 37 and, when the ramp voltage drops to apredetermined value, amplifier U2:C goes “high” and provides a signalcoupled through diode D8 to the PULSE GEN circuit. When a “high” signalis imposed on the negative (−) terminal of amplifier U2:B, the outputsignal of U2:B goes “low” shutting off an output signal from power triacQ1 and motor 32 stops. Accordingly, when the ramp voltage at conductors36, 37 increases slightly above a dropout value, the CUTOFF circuitoutput at amplifier U2:C goes low. This allows the ramp voltage toresume normal output to control the motor 32 through the triac Q1.

The CUTOFF circuit, including amplifier U2:C, is operably connected to ajumper JP1 in a MIN SPEED circuit as shown in FIG. 2. The discussionhereinabove regarding the CUTOFF circuit assumes that the jumper JP1 isopen. When jumper JP1 is closed, control system supply voltage iscoupled through diode D11 to the negative input terminal of amplifierU2:C. The output signal of amplifier U2:C is then forced “low”regardless of the ramp voltage input to amplifier U2:C and therefore thepulse generator is not shutoff due to the CUTOFF circuit. The MIN SPEEDcircuit of the control system 20 includes resistors R19, R22, R23, R28,R46, diodes D9 and D11 and amplifier U2:D. When the jumper JP1 is open,the output of the MIN SPEED circuit is low at the output of amplifierU2:D and the adjustable resistor R22, which is operable to adjust theminimum speed of the motor 32, is inoperable. When JP1 is closed, theCUTOFF circuit previously described is disabled through diodes D11 andresistor R46 and the MIN SPEED circuit is enabled. Adjustment of theminimum speed resistor or potentiometer R22 enables the motor 32 to beset to run from approximately 180 rpm to 620 rpm, for example. The motorminimum speed will hold even though there may be a zero differencebetween supply air and return air temperatures as sensed by the sensorsR9 and R11.

The aforementioned HEAT/COOL SELECT circuit includes a cooling conditioninput circuit including resistors R31 and R45, diodes D7 and D14 andcapacitor C11. A 24 volt AC signal on the aforedescribed circuit willdeactivate motor 32 by deactivation of triac Q1. This signal overridesany signals produced by the sensors R9 and R11. Consequently, when thecontrol system 20 is connected to the medium speed winding of a motor,such as the motor 32, and the conventional control system for the motorapplies power to the high speed winding and the 24 VAC COOL signal isprovided at terminals P4 and P6 only the desired motor winding will beenergized. However, when the thermostat is satisfied in the space beingcooled and a signal is removed from terminals P4 and P6 the controlsystem 20 will be operable to energize the motor 32 at the medium speedwinding and gradually reduce the motor speed as the temperaturedifference between the sensors R9 and R11 decreases.

Conversely, when a 24V AC HEAT input signal is provided at the HEAT/COOLSELECT circuit, the COOL RAMP circuit is disabled and only thetemperature of sensor R9 rising above the temperature of sensor R11 willaffect motor speed. The sensor R9 temperature, when below the sensor R11temperature, will maintain the HEAT/COOL RAMP GEN circuit at its minimumvoltage. Motor 32 will either then be at zero speed or a minimum speeddepending on the selection of the connection of jumper JP1 for cutoff orminimum speed. The heat input side of the HEAT/COOL SELECT circuitincludes capacitor C12, diodes D12, D13 and resistors R41 and R42.

Lastly, the control system 20 includes a SENSOR PROTECTION circuitincluding resistors R47, R48, R49, R50, R51, capacitors C1, diode D15and amplifier U4:A. A positive input signal to amplifier U4:A of theSENSOR PROTECTION circuit is provided by the ramp output voltage signaland the protection circuit negative input to amplifier U4:A is connectedto a reference voltage available from resistors R49 and R50. When theramp output voltage exceeds the reference voltage, the output signal ofamplifier U4:A goes high and this DC voltage signal is connected to theopto-isolator U3 through diode D15. This action causes the triac Q1 tobe on full at all times and avoid the possibility of motor lockup.

Still further, there are three ways for the ramp output voltage signalto exceed the reference voltage signal at amplifier U4:A, whichreference is established by resistors R49 and R50 namely (1) if eitherof the sensors R9 or R11 are open, (2) if both of sensors R9 and R11 areopen or are shorted, or (3) if the design parameter for the temperaturedifference between sensors R9 and R11 has been exceeded. If any of theabove noted conditions occurs the motor 32 will be fully on until thecondition goes back to the system normal mode of operation or power isremoved from the control system 20.

An alternate embodiment of a control system in accordance with theinvention will now be described in conjunction with FIGS. 3 and 4. Incertain air conditioning systems it may be necessary to monitor a changein cooling air temperature and heating air temperature at differentlocations in the air conditioning system and the reference or return airsensor may be required to be mounted in a return air duct to monitortemperature in a third location. A major advantage of having theflexibility of being able to choose the location of the temperaturesensors is with regard to certain installations wherein, for example,during a cooling phase of operation air is routed through a differentduct than for the routing of air during heating operation. Stillfurther, in other applications the heating/cooling equipment may bearranged such that the location of the so-called supply air sensor maybe suitable for the heating mode of operation but not the cooling modeor vice versa.

By way of example, and referring to FIG. 4, there is illustrated avertical or updraft air conditioning system 110 which includes duct orcabinet 111. Cabinet 111 is mounted on a return air plenum 112 wherebyair being returned from an air conditioned space enters the system 110and flows upward over surfaces of a heat exchanger 114 and then furtherupward through an evaporator coil or air cooling heat exchanger 116before being discharged into a supply air plenum 118 for distributionthrough suitable supply air ducts 120, 122 and 124 for example. Themotor and blower or fan 32, 33 for the system 110 is shown in onepreferred location in plenum 112 in the somewhat schematic illustrationof FIG. 4, but may also be located, alternatively, in the cabinet 111,for example.

A control system 200 is illustrated in FIGS. 3A, 3B and 3C which isadvantageous for use with the air conditioning system 110 of FIG. 4. Thesystem 110 of FIG. 4 is provided to illustrate that a typical locationof a return air sensor 130 would be in the return air duct or plenum112. A heated air sensor 132 should be disposed just downstream in thedirection of airflow through the system 110 of the heat exchanger 114for sensing the temperature of heated air, and a third or cooling airsensor 134 is shown located just downstream, in the direction ofairflow, from the evaporator or cooling coil 116. With this arrangementmore accurate and timely readings of the heated air temperature and thecooled air temperature is provided even though the flowpath for the airduring the heating mode or the cooling mode is not through separateducts in the exemplary system 110.

Referring now to FIG. 3C, the control system 200 includes a POWER SUPPLYcircuit substantially like the power supply for the control system 20.Power at 120 volts AC may be applied at terminals P1 and P3 oralternatively 24 volt AC power may be applied at terminals P1 and P2. Asindicated in FIG. 3C, a jumper JP2 is applied if 120 volt AC power isconnected to the control system. Motor 32 is connected between terminalsP1 and P4 also, as indicated. The POWER SUPPLY circuit is made up ofcapacitors C60 and C50, resistor R200, diodes D50 and D80 and bridgecircuit BR1. Bridge circuit BR1 is a set of four diodes which cause afullwave bridge output signal to be developed. Since the voltagedeveloped falls to near zero every half cycle, the output is used tosynchronize a pulse generator by resetting capacitor C140 every halfcycle. A RESET circuit including diode D70, resistors R280, R420, R350,capacitor C100, resistor R460, resistor R410, resistor R400, amplifierU2:A1 and diode D110 provides an output pulse to amplifier U2:B1 at thecorrect moment on each cycle.

Control system 200 also includes a PULSE GEN circuit, as shown in FIG.3C, comprising resistors R510, R600 and R610, diode D150, amplifierU2:B1 and capacitors C130, C140 and C110. A voltage signal from the rampcircuits shown in FIG. 3A and to be described further herein is providedvia reference terminal 212 in FIG. 3A to reference terminal 214 in FIG.3C. This voltage is applied to the positive terminal of amplifier U2:B1and a reference voltage provided through resistors R510 and R600 isapplied to the negative terminal of amplifier U2:B1. When the rampvoltage at reference terminal 214 exceeds the reference voltage,amplifier U2:B1 provides a “high” output signal. Capacitor C140 connectsto the ramp voltage signal imposed on reference terminal 214 and asawtooth waveform results at the positive terminal of U2:B1.Accordingly, the output signal of amplifier U2:B1 is a square wave whosewidth varies as the ramp voltage imposed on reference terminal 214varies.

A POWER OUTPUT circuit of the control system 200 includes resistorsR490, R570, amplifier or opto-isolator U60 and power triac Q1 as well ascapacitor C120. The aforementioned squarewave output signal from theamplifier U2:B1 is connected to capacitor C110. A voltage pulse isformed by capacitor C110 and diode D150 to the input diode ofopto-isolator U60. When the aforementioned input diode of opto-isolatorU60 conducts, an output triac of the opto-isolator U60 turns “on” whichcauses current to flow into the gate of power triac Q1. Motor 32 isconnected to the power triac Q1 at terminal P4 and when current flowsthrough the triac, the motor is energized to drive fan or blower 33. Asnubber resistor-capacitor combination comprising resistor R570 andcapacitor C120 are connected to the power triac Q1 to protect the triacfrom unexpected line voltage surges.

Referring now to FIG. 3B also, a fan motor speed CUTOFF circuit is shownincluding resistors R400, R150, R160, R180, R90, R120, diode D300, diodeD200 and amplifier U2:C1. The negative terminal of amplifier U2:C1 isconnected to a HEAT/COOL RAMP circuit shown in FIG. 3A through resistorR400 via conductor 221. When the COOL RAMP circuit output voltage dropsto a predetermined level, as determined by the reference voltage at thepositive terminal of amplifier U2:C1, this amplifier provides an outputsignal to diode D200 and the PULSE GEN circuit by way of conductor 223which is coupled to the negative input terminal of amplifier U2:B1. Whena “high” output signal is applied via conductor 223 to amplifier U2:B1,the output signal of amplifier U2:B1 goes “low” shutting down the outputsignal of opto-isolator U60 and power triac Q1 thereby deenergizingmotor 32. When the HEAT/COOL RAMP voltage signal from conductor 221increases slightly above a so-called dropout voltage, the output signalfrom amplifier U2:C1 goes low and allows the ramp voltage signal toresume normal action to control the motor speed through the power triacQ1. It should be noted that the CUTOFF circuit just described has ahysteresis equivalent to approximately 10 rpm on motor 32.

The above description with respect to the CUTOFF circuit assumes thatthe jumper JP3, FIG. 3A, is in an open condition. When jumper JP3 isclosed, the supply voltage provided thereby enables a reference voltageto the noninverting input of amplifier U2:D1, the output voltage ofwhich is coupled through diode D140 to the output conductors of theHEAT/COOL RAMP and HEAT RAMP circuits and the reference terminals 212,214, which determines the motor speed by biasing amplifier U2:B1 aspreviously described. By adjusting an adjustable resistor R540 of a MINSPEED circuit, FIG. 3A, the minimum speed of the motor 32 can be preset.

However, when jumper JP3 is open, switch U5:A1 is connected across theJP3 contacts. Switch U5:A1 is energized through resistor R450 whichconnects to a 24 volt AC HEAT signal of the HEAT/COOL SELECT circuit,FIG. 3A, by way of reference terminals 225 and 227 and by way of diodeD170 resistor R690, diode D190, capacitor C150 and resistor R450. Withthis arrangement, when a 24 volt AC HEAT signal is present, the MINSPEED circuit is energized and if the MIN SPEED circuit is energized,the CUTOFF circuit, FIG. 3B, is deenergized by way of resistors R580 andR400. Accordingly, when a signal is applied to the 24 volt AC HEATinput, switch U5:A1 is immediately switched on and this action shortsjumper JP3 from resistor R530 to positive 12 volts DC. A bias voltage isapplied to positive pin of amplifier U2:D1 and the output of amplifierU2:D1 is applied at the reference terminal 212 through diode D140.Therefore, as the control system 200 is operated in conjunction with theair conditioning system 110 wherein a signal indicating a heat mode ofoperation is applied, the motor 32 runs at a minimum speed to providebetter air circulation surrounding sensors 130, 132 and 134.

Adjustment of the minimum speed MIN ADJUST resistor R540 enables themotor speed to be set from approximately 180 rpm to 620 rpm. The minimumspeed of the motor 32 will hold at its designated RPM even though theremay be no difference between supply air and return air temperatures.

Referring further to FIG. 3A, the HEAT/COOL SELECT circuit is-operableto provide a 24 volt AC input signal at 24 VAC COOL across terminals P6and P7 a and imposed on diode D200 resistors R700, R710, diode, D210 andcapacitor C160. When 24 volt AC power is applied across terminals P6 andP7 a, motor 32 is turned off by deactivation of power triac Q1 due tothe application of an output signal at reference terminal 229 which isconnected to reference terminal 231 FIG. 3C. In other words, when avoltage is applied to reference terminals 229, 231 and the negativeterminal of amplifier U2:B1 the output signal of amplifier U2:B1 goes“low” and causes opto-isolator U60 and power triac Q1 to shut off powerto motor 32. A signal as described above applied at reference terminal229 overrides signals provided by return air sensor 130 and supply airsensors 132 and 134. Accordingly, the control system 200 is alsooperable to avoid supplying power to both the high speed winding and themedium speed winding of the motor 32 when the thermostat for the airconditioning system 110 has called for operation in the cooling mode andthe motor is being separately controlled by the conventional motorcontrol system to operate at a high speed. However, as with the controlsystem 200 when a control signal is removed across terminals P6 and P7a, the control system 200 will assume control over the motor 32 and willgradually decrease the speed of the motor as determined by thedifference in temperatures sensed by the sensors 130 and 134.

Referring further to FIG. 3A, when a 24 volt AC signal is applied acrossterminals P5 and P7 a and diode D170, resistor R690, diode D190,capacitor C150 and diode D180, the COOL RAMP circuit is disabled by wayof reference terminal 233 which is connected to reference terminal 235in FIG. 3A. Under these operating conditions only temperatures risingabove the sensor 132 temperature compared to the return air sensor 130will affect motor speed. Temperatures sensed by the heat sensor 132 andcool air sensor 134, if less than the temperature sensed by the returnair sensor 130, will only maintain a HEAT RAMP circuit output voltage atits minimum. This will cause the motor 32 to operate at zero rpm or atits minimum speed, depending on whether a cutoff or minimum speed modeis chosen.

Referring now to FIG. 3B, an OVERVOLTAGE RAMP AND SENSOR PROTECTIONcircuit is provided which includes capacitor C70, resistors R260, R270,R310 and R340, amplifier U4:A1, diode D90 and resistor R330 for the HIGHRAMP output. The OVERVOLTAGE RAMP AND SENSOR PROTECTION circuit furtherincludes resistors R470, R500, R440, R480, amplifier U4:B1 and diodeD120, for the ZERO RAMP. Resistors R550, R620, R560, R590, amplifierU4:C1 and diode D130 comprise the circuit of a RETURN AIR SENSE OPENoutput. Resistors R660, R680, R650, R670, amplifier U4:D1 and diode D160comprise the circuit for the RETURN AIR SENSOR SHORT output. The purposeof these circuits is to cause the motor 32 to be driven at full speed ifeither of the sensors 130, 132 or 134 is in an open or a shortedoperating condition, or if the HEAT/COOL RAMP is at zero or greater thanthe ramp voltage boundaries.

Referring further to FIG. 3B, amplifier U4:A1 receives an input signalon its positive terminal by way of conductor 237 which is connected tothe HEAT/COOL RAMP of FIG. 3A. An output signal from amplifier U4:A1goes high when the COOL RAMP or HEAT RAMP circuit output signals equalor exceed a differential temperature trip point. In fact, amplifiersU4:A1, U4:B1, U4:C1 and U4:D1 are all operable, when providing a highoutput signal, to cause motor 32 to run at full speed. The outputsignals from any one of these amplifiers is conducted via referenceterminals 239 and 241 to the opto-isolator U60.

Referring further to FIG. 3B, an output signal from amplifier U4:B1 goeshigh when the output from the HEAT RAMP or COOL RAMP circuits goes tozero. This occurrence would be the result of the cool sensor 134 beingshorted or the heat sensor 132 going to an open condition. The outputsignal from amplifier U4:A1 goes high when the output signal from theHEAT RAMP circuit or the COOL RAMP circuit goes high. This occurs whenthe cool sensor 134 has an open circuit condition or when the heatsensor 132 experiences a shorted condition. Still further, the outputsignal from amplifier U4:C1 goes high when the return air sensor 130 isin an open condition and the output signal from amplifier U4:D1 goeshigh when the return air sensor is shorted. A signal from the return airsensor 130 is supplied to amplifiers U4:C1 and U4:D1 via referenceterminals 243 and 245. Reference terminals 247 and 249, FIG. 3A, arealso connected to reference terminal 245 and impose signals onamplifiers U1:C1 and U3:B1.

Referring further to FIG. 3A, the HEAT RAMP circuit receives a variablevoltage signal from the heat sensor 132 by way of a buffer amplifierU3:C1. The output of the HEAT SENSE circuit, the junction of resistorsR720 and R430, is connected to the HEAT RAMP circuit through resistorR390 and buffer amplifier U3:C1. Amplifier U3:A1 is a differentialamplifier and its output voltage is determined by the difference betweenthe heat sensor voltage output signal and the return air sensor voltageoutput signal which is the output signal from the junction of resistorsR190 and R210 as imposed on reference terminal 249. Amplifier U3:B1 isalso a buffer amplifier for the return air sensor voltage output signal.A variable voltage output signal from amplifier U3:A1 and diode D100 isthus imposed on reference terminals 212 and 214 through resistors R580and R630.

Referring still further to FIG. 3A, the COOL RAMP circuit includesresistors R20A, R30A, R50A, R70, R80, R10A, R11A, capacitors C10A andamplifiers U1:B1, U1:C1 and U1:D1 as well as diode D10A. Capacitors C10Aand C90 minimize the effect of a step function at the output of the coolsensor 134 or heat sensor 132 to prevent motor lockup and also tominimize unwanted electrical noise from entering the circuit. The outputsignal from the cool sensor 134 is connected to the COOL RAMP circuitthrough resistor R70 to buffer amplifier U1:B1 whose output is imposedon amplifier U1:D1. Amplifier U1:D1 is a difference amplifier whoseoutput signal is determined by the difference between the cool sensorvoltage output signal and the return air sensor voltage signal at thejunction of resistors R190 and R210. Amplifier U1:C1 is a bufferamplifier for the output signal of return air sensor 130. The outputsignal from the COOL RAMP circuit is by way of amplifier U1:D1 throughdiode D10A to conductor 221 and to reference terminal 212 by wayresistors R580 and R630. The output signal from the junction ofresistors R60 and R100 is also imposed on amplifier U1:A1 by way ofreference terminals 253 and 255 to disable the HEAT RAMP circuit whenthe system 200 is operating in a cooling mode. Conversely, the output ofthe heat sensor 132, as measured at the junction of resistors R720 andR430, is imposed on reference terminals 257 and 259 and amplifier U3:D1to disable the COOL RAMP circuit. The ramp circuits will generate avoltage signal as the differences between the return air sensor voltageand the heat sensor voltage or cool sensor voltage pass outside of adead band of approximately 5° F. The cooling temperature signal outputmust be below the referenced temperature by about 2.5° F. and thetemperature signal from the heat sensor must be above the referencetemperature by about 2.5° F. When either condition exists, the rampvoltage signal imposed on terminal 221 will increase starting justoutside the deadband.

The operation of the control systems 20 and 200 to vary the speed of afan motor for a forced air air conditioning system for the advantageouspurposes set forth herein is believed to be understandable to those ofordinary skill in the art based on the foregoing description. Acorrelation table of the components of the systems 20 and 200 is setforth hereinbelow. Certain ones of the circuit components shown in thedrawing and included in the correlation table are not discussed indetail but are believed to be understandable to those of ordinary skillin the art. Preferred values and commercial part numbers for certaincomponents are identified also.

CORRELATION TABLE COMMERCIAL ITEM PART NO. VALUE C1 .1 μF C2 100 μF 63 VC4 .22 μF C5 .1 μF C6 .1 μF C7 .1 μF 250 V C8 0.1 μF C9 .1 μF C10 .1 μFC10A 0.47 μF C11 10 μF 25 V C12 10 μF 25 V C13 .47 μF C14 .47 μF C20 0.1μF C30 0.1 μF C40 0.1 μF C50 100 μF 63 V C60 0.1 μF C70 0.1 μF C80 0.1μF C90 0.47 μF C100 0.22 μF C110 0.1 μF C120 .1 μF 250 V C130 0.1 μFC140 0.1 μF C150 10 μF 25 V C160 10 μF 25 V D1 IN4148 D2 IN4003 D3IN4148 D4 IN4148 D5 IN4742A D6 IN4148 D7 IN4148 D8 IN4148 D9 IN4148 D10IN4148 D10A IN4148 D11 IN4148 D12 IN4148 D13 IN4148 D14 IN4148 D15IN4148 D40 IN4148 D50 IN4003 D60 IN4148 D70 IN4148 D80 IN4742A D90IN4148 D100 IN4148 D110 IN4148 D120 IN4148 D130 IN4148 D140 IN4148 D150IN4148 D160 IN4148 D170 IN4148 D180 IN4148 D190 IN4742A D200 IN4148 D210IN47742A D300 IN4148 Q1 BTA 6 R1 1 M R2 475K R3 10K R4 475K R5 1 M R6 1M R7 26.7K R8 1 M R9 10K THERMISTOR R10 R10A R11 10K THERMISTOR R11A R1210K R13 825K R14 1 M R15 750. 1/2 W R16 1 M (RP2.4) R17 1.24K R18 825KR19 25.5K R20 1 M R20A 1 M R21 499K R22 5K R23 634K R23A R24 1 M R24AR25 1 M R26 2.74 M R27 100K R28 22.1K R29 44.2K R30 470 1/2 W R30A R317.5K R32 510 2 W, METAL OXIDE R32A R33 1 M R34 1 M R35 10K R36 6.04K R37499 R37A R38 10.5K R39 10K R40 200K R41 7.5K R42 100K R43 10K R44 20.5KR45 10K R46 100K R47 1 M R48 1 M R49 3.48K R50 10K R50A 10K R51 2K R707.5K R71 100K R80 { } R90 200K R100 R120 100K R130 R140 1 M R150 1 MR160 10.5K R170 1 M R180 10K R190 R200 6K 2 W R210 10K R220 1 M R250 1 MR260 10K R270 3.48K R280 R290 R300 1 M R310 1 M R330 2K R340 1 M R3502.74 M R360 R380 R390 1 M R400 1 M R410 1 M R420 100K R430 R440 1 M R45010K R460 20.5K R470 R480 1 M R490 470 1/2 W R500 R510 10K R520 10K R53063.4K R540 5K R550 R560 1 M R570 510 2 W. METAL OXIDE R580 25.5K R590 1M R600 6.04K R610 499 R620 R630 22.1K R640 44.2K R650 1 M R660 R670 1 MR680 R690 7.5K R700 7.5K R710 100K R720 U1:A LM2902 U1:A1 LM2902N U1:BLM2902 U1:B1 U1:C LM2902 U1:C1 LM2902N U1:D LM2902 U1:D1 LM2902 U2:ALM2902 U2:A1 LM2902 U2:B LM2902 U2:B1 LM2902 U2:C LM2902 U2:C1 LM2902U2:D LM2902 U2:D1 LM2902 U3 MOC3052N U3:A1 LM2902 U3:B1 LM2902N U3:C1U3:D1 LM2902N U4:A LM2904 U4:A1 LM2904 U4:B1 LM2902N U4:C1 LM2902N U4:D1LM2902N U5:A1 CD4066 U60 MOC3052NAlthough preferred embodiments of the invention have been described indetail herein, those skilled in the art will also recognize that varioussubstitutions and modifications may be made without departing from thescope and spirit of the appended claims.

1. A control system for controlling the speed of a fan motor for aforced air conditioning system operable in an air heating mode and anair cooling mode, respectively, to vary motor speed during operation ofsaid air conditioning system to minimize temperature stratification andheating or cooling exchange losses in said air conditioning system, saidcontrol system comprising: a first temperature sensor disposed in areturn air flowpath of air returning from an enclosed space to said airconditioning system; a second temperature sensor located downstream ofat least one heat exchanger of said air conditioning system formeasuring the air temperature at the location of said second sensorcompared with the air temperature at the location of said first sensor;a first control circuit including first and second amplifiers connectedto each other and to at least one of said sensors and third and fourthamplifiers connected to each other and to at least one of said sensorsfor producing a variable output signal dependent on the difference intemperatures sensed by said first and second sensors and dependent onwhether said air conditioning system is operating in an air heating modeor an air cooling mode; a power output circuit for controlling the speedof said motor as a function of said output signal to vary the speed ofsaid motor in accordance with the difference in temperatures sensed bysaid first and second sensors; and a heating operation and coolingoperation selection circuit operably connected to said power outputcircuit and said first control circuit.
 2. The control system set forthin claim 1 wherein: said power output circuit includes an opto-isolatorand a triac operably connected to said motor and to a pulse generatorcircuit and operable to vary the speed of said motor in accordance witha pulse signal generated by said pulse generator circuit.
 3. The controlsystem set forth in claim 1 including: a cutoff circuit operable when avoltage signal generated by said first control circuit decreases to apredetermined value to cause said power output circuit to de-energizesaid motor.
 4. The control system set forth in claim 3 including: aminimum speed circuit operably connected to said cutoff circuit to causesaid power output circuit to operate said motor at a predeterminedminimum speed.
 5. The control system set forth in claim 4 including: anadjustable resistor operably disposed in said minimum speed circuit foradjusting the minimum speed of said motor.
 6. The control system setforth in claim 5 wherein: said resistor is operable to adjust theminimum speed of said motor to operate in a range from approximately 180rpm to 620 rpm.
 7. The control system set forth in claim 1 including: asensor protection circuit operably connected to said first controlcircuit and said power output circuit for causing said power outputcircuit to operate said motor at full speed when an output signal fromsaid first control circuit exceeds a predetermined value.
 8. A controlsystem for controlling the speed of a fan motor for a forced air flowair conditioning system operable in a heating mode and a cooling modeand including means forming an air flowpath therein, said control systembeing operable to vary motor speed during operation of said airconditioning system to minimize temperature stratification in said airflowpath and to minimize heat exchange losses from at least one heatexchanger disposed in said air flowpath, said control system comprising:a first temperature sensor disposed in a return air part of said airflowpath for measuring the temperature of air returning from an enclosedspace to said air conditioning system; a second temperature sensorlocated in said air flowpath downstream of said at least one heatexchanger for measuring the air temperature at the location of saidsecond sensor compared with the air temperature at the location of saidfirst sensor; a first control circuit including first and secondamplifiers connected to each other and to at least one of said sensorsand third and fourth amplifiers connected to each other and to at leastone of said sensors for producing a variable output signal dependent onthe difference in temperatures sensed by said first and second sensorsand dependent on whether said air conditioning system is operating in anair heating mode or an air cooling mode; a second control circuitconnected to said first control circuit and to a power output circuitfor varying the speed of said motor as a function of the difference intemperatures sensed by said first and second sensors to minimizetemperature stratification in said air flowpath and to minimize heatexchange losses by continuing a progressively changing flow of airacross said at least one heat exchanger and through said air flowpathduring at least one of startup and shutoff of heat transfer to or fromsaid at least one heat exchanger; and a heating operation and coolingoperation selection circuit operable for controlling the speed of saidmotor in accordance with operation of said air conditioning system insaid heating mode and said cooling mode, respectively.
 9. The controlsystem set forth in claim 8 wherein: said power output circuit includesan opto-isolator and a triac operably connected to said motor andoperable to vary the speed of said motor in accordance with a pulsesignal.
 10. The control system set forth in claim 8 including: a cutoffcircuit operable when a ramp voltage signal decreases to a predeterminedvalue to cause said power output circuit to de-energize said motor. 11.The control system set forth in claim 10 including: a minimum speedcircuit operably connected to said cutoff circuit to cause said poweroutput circuit to operate said motor at a predetermined minimum speed.12. The control system set forth in claim 8 including: a sensorprotection circuit operably connected to said first control circuit andsaid power output circuit for causing said power output circuit tooperate said motor at full speed when an output signal from said firstcontrol circuit exceeds a predetermined value.